Generation and Progress Behavior of Strain-Induced Abnormally Large Grains in Superalloy 718

Herein, the behavior associated with strain-induced abnormally large grains in superalloy 718 was investigated using compression testing and subsequent heat treatment below the δ-phase solvus temperature of 980 °C. The nuclei of abnormally large grains included some grains that were newly recrystallized through nucleation. Abnormally large grains were caused by a difference in intragranular misorientation between the dynamic recrystallized grains and the deformed matrix. The initiation of abnormally large grains was retarded in response to a decrease in plastic strain, leading to the formation of microstructures consisting of larger grains with a more complex morphology. Grain boundaries migrated locally in a direction perpendicular to, or predominantly in a direction parallel to, the Σ3 {111} twin boundaries, along with the high-order twin formation. To determine the direction parallel to the Σ3 {111} twin boundaries, the regions where parent grain and dissimilar twin growth occurred in the same direction along their twin boundaries were analyzed using {110} PFs. The growth direction consisted of only one set of common 〈110〉 axes such that the arrangement of atoms was consistent. Owing to multiple twinning, abnormally large grains seemed to evolve in varying growth directions.